This invention relates to an image reading device, and more particularly to an image reading device having a moire depressing unit for depressing occurrence of moire of an image to be read.
A line image sensor comprising plural photoelectric transducers such as charge-coupled devices has been generally utilized as an image reading device for reading an original image in a fax machine, a digital copying machine or the like. When an original image to be printed with dots is read by this type of image reading device, an output signal is frequently provided with moire, which does not exist in the original image. It has been known that the moire occurs in a case where an original image is dot-printed, and a pitch interval of dots is near to a sampling pitch of an image sensor for reading the original image. In order to depress the moire, there has been conventionally utilized an optical low path filter through which signals of frequency components homogeneously contributing to the moire are eliminated. In such a low path filter, since all signals are homogeneously smoothed, signals of high frequency components are also eliminated by the low path filter even though they are parts of the image components and do not contribute to the moire. As a result, edge portions of the original image are blurred and this resolution of the image is reduced.